"/dev/**** No such device" error coming even with valid charachter device - linux

Question

I am writing a kernel driver in drivers/char/new_driver.c .
This new driver registers two new devices /dev/device1 and /dev/device2 by calling misc_register() api.
error = misc_register(&device1);
error = misc_register(&device2);
static struct miscdevice device1 = {
MISC_DYNAMIC_MINOR,
"device1",
&device1_fops
};
static struct miscdevice device2 = {
MISC_DYNAMIC_MINOR,
"device2",
&device2_fops
};
When I load the module I see that 2 two devices are getting created properly in /dev/device1 and /dev/device2
But when I try to write/read operation on this device it gives error saying that there is "No such device " .
Any idea what causes this type of error ? Anything missing in the driver code ?

Answer 1

I still believe that this is permission issue. I am attaching the sample code (without any real implementation) skeleton. Please have a look into it.
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/module.h>
static int sample_open(struct inode *inode, struct file *file)
{
pr_info("Maverick: %s func :\n",__func__);
return 0;
}
static int sample_close(struct inode *inodep, struct file *filp)
{
pr_info("Maverick: %s func :\n",__func__);
return 0;
}
static ssize_t sample_write(struct file *file, const char __user *buf,
size_t len, loff_t *ppos)
{
pr_info("Maverick: %s func :\n",__func__);
return len; /* Not doing anything with the data */
}
static ssize_t sample_read(struct file *file, char __user *buf,
size_t len, loff_t *ppos)
{
pr_info("Maverick: %s func :\n",__func__);
return len; /* Not do anything with the data */
}
static const struct file_operations sample_fops = {
.owner = THIS_MODULE,
.write = sample_write,
.read = sample_read,
.open = sample_open,
.release = sample_close,
};
struct miscdevice sample_device = {
.minor = MISC_DYNAMIC_MINOR,
.name = "device1",
.fops = &sample_fops,
};
static int __init misc_init(void)
{
int error;
error = misc_register(&sample_device);
if (error) {
pr_err("can't misc_register :(\n");
return error;
}
pr_info("Maverick: %s func :\n",__func__);
return 0;
}
static void __exit misc_exit(void)
{
misc_deregister(&sample_device);
pr_info("Maverick: %s func :\n",__func__);
}
module_init(misc_init)
module_exit(misc_exit)
MODULE_DESCRIPTION("Sample Misc Driver");
MODULE_AUTHOR("Vinod Maverick <vinodmaverickr007#gmail.com>");
MODULE_LICENSE("GPL");
Then compile the driver:
make
sudo insmod misc_sample.ko
ilab#SSID-iLBPG3:~/vinod/ldd$ echo "hello" > /dev/device1
-bash: /dev/device1: Permission denied
ilab#SSID-iLBPG3:~/vinod/ldd$ sudo su
root#SSID-iLBPG3:/home/ilab/vinod/ldd# echo "hello" > /dev/device1
However I have not any code inside the read/write callback but still you can see the dmesg without any error
root#SSID-iLBPG3:/home/ilab/vinod/ldd# dmesg
[2903599.416005] Maverick: misc_init func :
[2903623.966281] Maverick: sample_open func :
[2903623.966292] Maverick: sample_write func :
[2903623.966295] Maverick: sample_close func :
root#SSID-iLBPG3:/home/ilab/vinod/ldd#

Answer 2

To anyone who still has this problem, I managed to fix it by changing my minor number from a crazy high number (999) to a more reasonable number (71). Here's my resulting miscdevice struct:
static struct miscdevice chrdev = {
.minor = 71,
.name = "tracefunc",
.fops = &chrdev_fops,
.mode = S_IRUGO,
};